Method of plating metallic surfaces



METHQD F PLAYING METALLIC SACES C. B. Boyett, San Antonio, Tex, assignor to Terry Ingustries, Inc, San Antonio, Tex., a corporation. of

exas

No Drawing. Application February 18, 1955 Serial No. 489,288

Claims. ca. 204-43 This invention relates to the field of metallurgy and particularly to a novel method of plating a metallic surface, theprimary object being to provide a method capable of assuring a proper and satisfactory joinder between the metallic surface and the plating material, thereby avoiding separation of the two materials during various uses to which the same may be placed.

Plating of metallic surfaces particularly through the well-known electrolytic process is for the most part satisfactory when certain metals, metal alloys or compounds are utilized. However, so far as I am aware, there has not heretofore been made known a satisfactory process of electroplating aluminum, magnesium and similar metallic materials or otherwise applying a plating material thereto for the reason that no suitable means has been developed for preventing oxidation of such base materials.

It is essential in order to properly unite the plating material with the metallic surface to be coated that the two metals be in intimate engagement, in complete absence of intermediate materials or substances, and therefore, the presence of the aforementioned oxides, or the utilization of other oxides to which the coating is bonded, has proved unsatisfactory.

It is an important object of the instant invention therefore, to provide a coating method that contemplates the usual procedure of cleaning by removal of greases and oxides from the surface to be coated but which includes, as a novel expedient, the additional step of preventing all possibility of oxidation prior to and during the plating step, whereby the coating joins directly with the coated surface rather than with an oxide or other separating medium.

Another important object of this invention is to provide a method of the aforementioned character capable of permitting satisfactory coating of aluminum and other metallic materials which are inherently incapable of permitting utilization of a reverse current in the electroplating process which is satisfactorily utilized with most metals to remove metallic oxides immediately prior to electrodeposition of the plating material thereon.

Many additional objects, relating not only to the method itself, but to the production of an improved end product, will be made clear or become apparent as the following specification progresses.

It is well known that in order to properly plate a metallic surface with such coatings as chromium, silver, nickel, brass and the like the surface being coated should be absolutely clean and free from all oxides and additionally, particularly where the plating is accomplished by electrolytic action, it is essential to remove grease and oxides from the metallic surface so as to provide a proper union of the two metals.

In the case of many metals to be coated, such as steel, the removal of oxides from the surface to be coated is relatively simple. And the surface may be deoxidized quickly and easily by merely traversing the plating solution as well as the article itself immersed therein with 2,893,931 Patented July 7, 1959 a reverse currentimmediately prior to carrying out the plating step.

In the case of aluminum, magnesium and the like, however, the utilization of the reverse current method results in an immediate formation of an oxide film, and whenthe plating step is carried out, the coating joins with the oxide rather than with the metallic surface itself as is desirable and necessary in order to present a satisfactory result.

Accordingly, many methods have heretofore been suggested for meeting the problem but in no instance has any suggested method been successful so far as elimihating, the intermediate layer of material such as aluminum oxide. Some methods heretofore proposed actually suggest the formation of an intermediate material such as zinc oxide to which the coating is joined. However, even in such methods the finished product is entirely unsatisfactory and of little practical value.

In accordance with the method of the present invention, therefore, it is contemplated that the surface of the metal to be plated, such as aluminum, be cleaned, deoxidized and protected in such manner as to assure intimate union between the aluminum and the plating material. As will hereinafter be made clear the aluminum, magnesium or other surface is properly cleaned and deoxidized through use of the method about to be described and such surface remains absolutely clean and therefore free of all oxides, until such timeas the same is completely coated with the plating material.

As a preliminary step in the, method it is advisable to follow the common practice of first removing all oils, greases and other foreign matter from the surface to, be coated. This, step may be carried out in any desired manner such as by a vaporized degreasing or honing. For example, trichloroethylene may be heated and the metallic surface subjected to the vapors emanated therefrom in the conventional manner.

The next steps in the process are advantageously carried out simultaneously to (a) remove all oxides, (b) properly hone the metallic surface and (0) provide a. coating thereon of such nature as to protectively prevent all further oxidation until such time as it is possible for the workmen to immerse the object in a bath of plating material.

Methods heretofore proposed have satisfactorily removed the oxides and many well-known processes have successfully been utilized to provide the necessary honing. However, oxidation commences immediately and before the plating step can be carried out. An oxide film forms thereon and the resultant coating joins with such film rather than with the metal itself.

Accordingly the surface should be cleaned by blasting and to this end, in order to also hone the same and produce the aforesaid protective coating, it is to be preferred that a wet blast be employed utilizing a suitable abrasive such as pumice.

Many abrasives for carrying outthe wet blast or liquid honing step are available on the open market and therefore the use of any suitable solution is satisfactory, provided that the abrasive or like material is held in suspension within a liquid. It is not difficult to maintain pumice in such suspended condition within water but if necessary when other materials are used, suitable substances may be employed to keep the same in suspension.

The material is applied with great force and in so doing all metallic oxides are immediately removed and the surfaces are honed to such extent as may be desired, depending upon the requirements relative to smoothness in the finished product.

In addition to removal of oxides and honing of the metal the material utilized in carrying out the Wet blast .many utilitarian purposes. be suggested that aluminum or aluminum alloys so coated may be used in the aircraft industry to provide air surfaces having minimum air resistanceand which will or liquid honing step will adhere to the surface and provide a substantially air impervious protective coating ,thereon which prevents subsequent oxidation, For a predetermined period of time, at least suflicient to permit the workmen to proceed to the next step, such coating .will seal the surface being coated and prevent air contact therewith so that oxidation does not take place, all by virtue of the cohesion of the pumice solution to the clean, honed surfaces. a

. The object is thereuponimrnersed within the plating solution utilized in the electrolytic process and in this respect it is'well understood that the nature of such electrolyte. will depend upon the type of coating to be applied. Chromium, nickel, brass and silver are commonly .usedandwhen'the same is traversed with an electric current a gas is produced which causes agitation of the solution of the bath around the object to be plated. Such agitation is utilized in the method of the instant invention in an advantageous manner to remove the aforesaid protective coating without, as a consequence thereof, subjecting the metallic article to oxidation. To

this end a current of an amperage is caused to flow through the electrolyte sufficient only to cause formation of a hydrogen, cyanide or other gas which will agitate the solution so that the protective coating is washed away while the article is immersed in the plating solution. The

amperage should be sufliciently low (as for example 1-1% amperes) to prevent any plating until the coating .manner equally as eflicient as in the case of conventional plating processes of other metals such as steel, and .the, resultant product will stand up under heat, bending stresses and other forces to which the same may be subjected, without peeling of the plating, cracking, blistering or other separation as has heretofore been experienced in this field. In this respect, it is to be pointed out that the process is especially adapted for placing a chromium coating upon aluminum thereby adapting the latter for By way of example it may therefore, develop little frictional heat. This is particularlyadvantageous in connection with modern day aircraft operating at high speeds and in order to utilize the lightweight and strong characteristics of aluminum it is necessary to thus protect the same against deterioration from the frictional heat that would otherwise be developed by such high speeds inabsence of the hard,

durable chrome surface thereon.

' It is to be pointed out further and by way of example .only that the utilization of aluminum for such purposes as cylinder walls in internal combustion engines has been .hampered because of the heat problems and therefore the use of the above method of coating aluminums will have many extended uses.

In summary therefore, it is recognized that steel and many other metals have heretofore been successfully.

electroplated through conventional methods, and that developed primarily for use with metals that cannot be satisfactorily plated by known procedures.

e The plating of-aluminum with chromium is given as ;a primary example because of the wide uses to which such finished product could be placed, and because aluminum presents one of the most aggravating problems in this field. When presently known methods are used, the chromium soon blisters, cracks or peels off even when the product is not under hard use. If subjected to bending forces, temperature changes, or other stresses and strains, it quickly deteriorates.

The reason for such poor results lies solely in the failure to attain a satisfactory union between the two metals. Since the chromium becomes bonded to the oxides on the aluminum surface during the electrolytic process, it does not properly unite with the aluminum itself. Hence, the oxides and the plating material will readily pull away from the coated surface. Further, such manner of plating the aluminum does not inhibit further oxidation of the aluminum after the coating process is complete, resulting in even more rapid breakdown. These results are equally as deleterious when other materials are placed on the aluminum after cleaning in. order to prevent formation of aluminum oxide since such materials form their own oxides to which the chromium becomes bonded.

Accordingly, by selecting a protective coating that will positively prevent oxidation, will not form its own oxides, and which can be readily washed away in the plating solution without contaminating or otherwise effecting the latter in an adverse manner, the problems above enumerated are fully met and there is presented a fast, inexpensive and highly effective method of handling even the most troublesome metals.

The uses to which a product of this type may be adapted are unlimited. The strong, light-weight properties of aluminum combined with the added strength, wear and corrosive characteristics of chromium opens up many'new fields of machine parts, aircraft production, engine design, automotive manufacture, household products and the like, not heretofore made possible through the efforts of those who have for many years tried to .meet the problem.

Having thus described the invention what is claimed as new and desired to be secured by Letters Patent is:

1. The method of successively cleaning, deoxidizing and plating the surface of a metal comprising the sequential steps ofv directing awet blast containing a suspension of a finely divided, cohesive abrasive against the surface to hone the latter until all foreign substances and oxides have been removed; continuing the wet blasting until a substantially air impervious layer of abrasive is formed on the surface to seal the latter from the ambient atmosphere; transferring the coated metal to a plating solution without interfering with said layer of abrasive on the metal and before any oxidation of the surface takes place;

' sive is pumice.

3. The method of'successively cleaning, deoxidizing and plating the surface of aluminum with chromium comprising the sequential steps of directing a wet blast containing a suspension of a finely divided, cohesive abrasive against the surface to hone the latter until all foreign substances and oxides have been removed; continuing the'wet blasting until a substantially air impervious layer of abrasive is formed on the surface to seal the latter from the ambient atmosphere; transferring the coated metal to a plating tank containing a chromium plate solution without interfering with said layer of abrasive on the metal and before any oxidation of the surface takes-place; traversing the solution with an electric current sufficient only to cause enough gas to be evolved from the solution 'to thereby dislodge the abrasive. on the surface; and after complete dislodgment of the abrasive,

5 '6 increasing the amperage of said current to plate chro- OTHER REFERENCES mium from said solution on the surface of the aluminum. Modem Electroplating, A. G. Gray, John Wiley and References Cited in the file of this patent Egg New York (1953) pages 157-158 and page UNITED STATES PATENTS 5 Principles of Electroplating and Eleetroforming, Blum 2,091,386 Viers Aug. 31, 1937 et 211., second edition, 1930, McGraw-Hill Book Com- 2,118,956 Wagner May 31, 1938 pany, 1:10., New York, page 143.

F E PATENTS 10 Websters International Dictionary, 2nd edition, un- 685,734 Great Bntam Jan. 7, 1953 abridged, Page 710,096 Great Britain June 9, 1954 

1. THE METHOD OF SUCCESSIVELY CLEANING, DEOXIDIZING AND PLATING THE SURFACE OF A METAL COMPRISING THE SEQUENTIAL STEPS OF DIRECTING A WET BLAST CONTAINING A SUSPENSION OF A FINELY DIVIDED, COHESIVE ABRASIVE AGAINST THE SURFACE TO HONE THE LATTER UNTIL ALL FOREIGN SUBSTANCES AND OXIDE HAVE BEEN REMOVED; CONTINUING THE WET BLASTING UNTIL A SUBSTANTIALLY AIR IMPREVIOUS LAYER OF ABRASIVE IS FORMED ON THE SURFACE TO SEAL THE LATTER FROM THE AMBIENT ATMOSPHERE; TRANSFERRING THE COATED METAL TO A PLATING SOLUTION WITHOUT INTERFERING WITH SAID LAYER OF ABRASIVE ON THE METAL AND BEFORE ANY OXIDATION OF THE SURFACE TAKES PLACE; TRAVERSING THE SOLUTION WITH AN ELECTRIC CURRENT SUFFICIENT ONLY TO CAUSE ENOUGH GAS TO BE EVOLVED FROM THE SOLUTION TO THEREBY DISLODGE THE ABRASIVE ON THE SURFACE OF THE METAL; AND AFTER COMPLETE DISLODGEMENT OF THE ABRASIVE INCREASING THE AMPERAGE OF SAID CURRENT TO PLATE THE SURFACE IN SAID SOLUTION. 